Attenuator

ABSTRACT

An attenuator includes a T-type two terminal pair network including first and second terminals, first, second and third circuits, wherein the first terminal receives an input signal to be attenuated, wherein the first circuit is connected between the first and second terminals, wherein the second circuit is connected between the first circuit and the second terminal and is connected to the first circuit via a node, wherein the third circuit is connected to the node, and a capacitor connected to the node, wherein the capacitance value of the capacitor is variable.

The present Application is a Divisional Application of U.S. patentapplication Ser. No. 12/289,823, filed on Nov. 5, 2008.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an attenuator, and more particularlyrelates to an attenuator having a circuit element that adjusts theattenuation characteristic of the attenuator.

2. Description of the Related Art

An attenuator has been known as a circuit having a function to attenuatea gain of an input signal, and is used for a cellular phone, forexample. In the cellular phone, for example, an attenuator is providedbetween an antenna that receives a signal and a low noise amplifier thatadjusts a gain of the received signal. In this case, the attenuator hasa role to attenuate the gain of the input signal so that the gain of thereceived signal may not exceed the dynamic range of the low noiseamplifier. Recently, it has been required to perform communications byusing a high frequency signal in wideband such as ultra widebandcommunications systems. Thus, an attenuator capable of handling widebandhigh-frequency signals has been required. However, in some cases, theattenuation characteristic of an attenuator may be sharply changeddepending on the frequency of an input signal under the influence of aparasitic element component of a circuit element which constitutes theattenuator. This is because the impedance of the parasitic elementcomponent changes relative to the frequency of the input signal. As thefrequency of the input signal increases, a change in the attenuationcharacteristic of the attenuator becomes large. If the amount ofattenuation of the gain of the input signal changes greatly depending onthe frequency of the input signal, the low noise amplifier that receivesan output signal from the attenuator has to have a complicated circuitconfiguration such that the amplifier can correspond to the change inthe gain of the output signal of the attenuator. Thus, it is importantto adjust the attenuation characteristic of the attenuator and to designthe attenuator capable of handling wideband high-frequency signals.

There are mainly two kinds of attenuator configurations. One is a Π-typeattenuator and the other is a T-type attenuator. In the Π-typeattenuator, circuit elements are connected in the form of a Π-typecircuit in a two terminal pair network. Meanwhile, in the T-typeattenuator, circuit elements are connected in the form of a T-typecircuit in a two terminal pair network. Japanese Patent ApplicationPublication No. 2000-286659 (Patent Document 1) discloses a techniquerelated to an attenuator in which the Π-type attenuator is combined withthe additional T-type attenuator. FIG. 11 shows the attenuator describedin Patent Document 1. This attenuator adjusts the amount of attenuationof an input signal by adjusting a value of a control voltage applied toa control terminal 1008 and a value of a bias voltage applied to a biasterminal 1021. For example, Patent Document 1 describes an attenuationtechnique in which while the bias voltage is applied to the biasterminal 1021 to drive PIN diodes 1004, 1006, 1010, 1012, and 1014, thevalue of the control voltage applied to the control terminal 1008 isincreased. Thereby, internal resistances in the PIN diodes 1010, 1012,and 1014 are decreased, while internal resistances in the PIN diodes1004 and 1006 are increased, so that the amount of attenuation of thegain of the input signal received at an input terminal is increased. Inother words, in the technique described in Patent Document 1, theattenuation characteristic of an attenuator 1000 is adjusted byadjusting the balance between the control voltage to be applied to thecontrol terminal 1008 and the bias voltage to be applied to the biasterminal 1021.

The present inventor has found that the conventional technique accordingto Patent Document 1 has the following problem. As described above, itis important to adjust the attenuation characteristic of an attenuator,and to design the attenuator capable of handling wideband high-frequencysignals. However, in the technique described in Patent Document 1, theattenuation characteristic of the attenuator is adjusted by the valuesof the voltages to be applied to the control terminal and the biasterminal. In this case, for example, a step-down circuit for adjustingthe value of the voltage to be applied to the bias terminal is required,so that the circuit scale of the attenuator is increased. In addition,since the attenuation characteristic of the attenuator is adjusted bythe voltage to be applied to the terminal in Patent Document 1, athermal noise and a shot noise may in some cases be mixed in an outputsignal of the attenuator, the thermal noise and the shot noise beingturbulence of a voltage signal caused by the random motion of electriccharges to be superimposed on the applied voltage. Since a receivingcircuit of communications equipment is not to handle a signal having ahigh gain, the noise component has a great influence on a signal. Forthis reason, it is required that the receiving circuit have a circuitconfiguration of not generating a noise as much as possible.

SUMMARY

An attenuator according to the present invention includes: a T-type twoterminal pair network including first and second terminals, first,second and third circuits, wherein said first terminal receives an inputsignal to be attenuated, wherein said first circuit is connected betweensaid first and second terminals, wherein said second circuit isconnected between said first circuit and said second terminal and isconnected to said first circuit via a node, wherein said third circuitis connected to said node; and

a capacitor connected to said node, wherein an amount of attenuation ofsaid input signal is adjusted by a capacitance value of said capacitor.This capacitor is a shunt capacitor.

This shunt capacitor shunts an input signal of the attenuator. A currentcomponent to be shunted increases or decreases depending on acapacitance value of the shunt capacitor, since the amount of currentflowing through the shunt capacitor is proportional to the capacitancevalue of the shunt capacitor. If a shunt capacitor having a largecapacitance value is connected, the current component to be shuntedbecomes large, and accordingly a current component that flows out froman output terminal of the attenuator decreases. If the current componentthat flows out from the output terminal of the attenuator decreases, theamount of attenuation of the gain of the input signal increases since again of an output signal of the attenuator decreases. Conversely, if ashunt capacitance having a small capacitance value is connected, thecurrent component to be shunted decreases, and a current component thatflows out from the output terminal of the attenuator increases. If thecurrent component that flows out from the output terminal of theattenuator increases, the amount of attenuation of the gain of the inputsignal decreases since a gain of an output signal of the attenuatorincreases. In this manner, according to the present invention, theattenuation characteristic of the attenuator is adjusted by using thecapacitance component. Since the scale of the capacitance element issmall as compared with a step-down circuit, the present invention canalso prevent the circuit scale of the attenuator from being increased.Furthermore, in the present invention, since the attenuationcharacteristic of the attenuator is not adjusted by applying a voltageitself to a terminal of the attenuator, the attenuator can prevent theoutput signal of the attenuator from being mixed with a thermal noiseand a shot noise.

The present invention exhibits the following effects: an attenuator thatadjusts the attenuation characteristics of the attenuator and has a goodattenuation characteristic can be designed; the circuit scale of theattenuator can be prevented from being increased; and an unnecessarynoise component can be prevented from being mixed with an output signalof the attenuator.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, advantages and features of the presentinvention will be more apparent from the following description ofcertain preferred embodiments taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 shows an attenuator according to a first embodiment of thepresent invention;

FIG. 2 is a graph for explaining an example of an attenuationcharacteristic of the attenuator;

FIG. 3 shows a high-frequency equivalent circuit of a MOS transistor;

FIG. 4 shows an attenuator according to a second embodiment of thepresent invention;

FIG. 5 shows a simulation result of the attenuation characteristic ofthe attenuator;

FIG. 6 shows an attenuator according to a third embodiment of thepresent invention;

FIG. 7 is a diagram for explaining a parasitic capacitance generated ina gate connection;

FIG. 8 shows an attenuator according to a fourth embodiment of thepresent invention;

FIG. 9 shows an attenuator according to a fifth embodiment of thepresent invention;

FIG. 10 shows a simulation result of an attenuator according to a sixthembodiment of the present invention;

FIG. 11 shows a conventional attenuator; and

FIG. 12 shows an attenuator relating to the attenuator according to thefourth embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be now described herein with reference toillustrative embodiments. Those skilled in the art will recognize thatmany alternative embodiments can be accomplished using the teachings ofthe present invention and that the invention is not limited to theembodiments illustrated for explanatory purposes.

First Example

Embodiments of the present invention will be described below withreference to the accompanying drawings. FIG. 1 shows an attenuator 100according to an embodiment of the present invention. The attenuator 100is a two terminal pair network (two-port circuit) that includes acircuit element between ports composed of terminals 101 and 103, andports composed of terminals 102 and 104. The attenuator 100 includesMetal-Oxide-Semiconductor (MOS) transistors 105 and 106 which are a kindof a field-effect transistor. The transistors 105 and 106 are connectedin series between the terminal 101 and the terminal 103, as examples ofa first circuit and a second circuit respectively. The attenuator 100further includes a MOS transistor 107 connected in shunt between the MOStransistors 105 and 106. A circuit 109 formed of the MOS transistors 105to 107 is a typical T-type attenuator formed of the MOS transistors.Further, a capacitance element 108 is connected in shunt between the MOStransistors 105 and 106, which are in the T-type attenuator. Since theMOS transistor 107 and the capacitance element 108 are shunt componentsin the two terminal pair network, they are also connected to aninterconnection that connects between the terminal 103 and the terminal104. A value of the capacitance element 108 can be set to 20 [fF], forexample. These MOS transistors are typically n-type MOS transistors,however, the MOS transistors may be p-type MOS transistors. Each of theMOS transistors 105 to 107 includes a gate terminal.

FIG. 2 shows the attenuation characteristic of the attenuator 100 inFIG. 1 and that of the circuit 109, i.e., the attenuator 100 in FIG. 1from which the capacitance element 108 is removed. “A” in FIG. 2 denotesthe attenuation characteristic of the attenuator (circuit 109), i.e.,the attenuator 100 described in FIG. 1 from which the capacitanceelement is removed. “B” in FIG. 2 denotes the attenuation characteristicof the attenuator 100. The vertical axis shows an amount of attenuation,i.e., the ratio of a gain of an output signal to a gain of an inputsignal of the attenuator (represented by decibel (dB)). The horizontalaxis shows frequencies of the input signals to the respectiveattenuators. As apparent from FIG. 2, the attenuator 100 having theattenuation characteristic B has a larger amount of attenuation than thecircuit 109 having the attenuation characteristic A. This is because theinput signal is shunted via the capacitance element 108, so that acurrent value of the output signal is lowered in the attenuator 100. Inthis manner, the attenuator 100, in which the capacitance element 108 isadded to the T-type attenuator of the circuit 109, can adjust theattenuation characteristic. Furthermore, the attenuator 100 having theattenuation characteristic B is capable of keeping the amount ofattenuation at an approximate constant value even if the frequency ofthe input signal increases. On the other hand, as the frequency of theinput signal increases, the amount of attenuation of the gain of theinput signal largely decreases in the circuit 109 having the attenuationcharacteristic A. A “to-be-used frequency band” shown in FIG. 2 showsthe frequency range of signals to be received in a receiving circuitrelating to communications that use a wide-band high-frequency signal,such as a UWB communications system. To take a specific example, assumea lower limit of the to-be-used frequency band is 3 GHz and an upperlimit thereof is 5 GHz. In this case, the attenuator 100 having theattenuation characteristic B keeps the amount of attenuation at anapproximate constant value regardless of the frequency of the inputsignal in the to-be-used frequency band, while the circuit 109 havingthe attenuation characteristic A has large decreases in the amount ofattenuation in the to-be-used frequency band as the frequency of theinput signal increases. For this reason, when the circuit 109 having theattenuation characteristic A receives a signal in the “to-be-usedfrequency band” in FIG. 2, the amount of attenuation largely variesdepending on the frequency of the received signal. Thus, in thecommunications using the wide band frequency signals, such as the UWBcommunications, the circuit configuration of a low noise amplifier thatis located in the latter part of the attenuator becomes complicated. Inthis manner, the attenuator 100 in which the capacitance element 108 isadded to the T-type attenuator of the circuit 109, can adjust theattenuation characteristic and improve the attenuation characteristic.

Here, the reason why the amount of attenuation of the attenuator changesdepending on the frequency of the input signal will be described. FIG. 3shows a high-frequency equivalent circuit of the MOS transistor that isone component of the attenuator 100 according to FIG. 1. The MOStransistor includes three electrodes of a source (S), a drain (D), and agate (G), and among terminals of the electrodes, a parasitic capacitancecomponent is present. For example, a capacitor 301 is a parasiticcapacitance generated between the gate electrode and the sourceelectrode of the MOS transistor; a capacitor 302, between the gateelectrode and a substrate; a capacitor 303, between the gate electrodeand the drain electrode; a capacitor 304, between the source electrodeand the substrate; a capacitor 306, between the drain electrode and thesubstrate. Furthermore, a resistance 305 is a resistance componentbetween the source electrode and the drain electrode. As describedabove, there are multiple, unavoidable parasitic capacitance componentsin the MOS transistor. Here, the attenuator 100 includes multiple MOStransistors as its circuit configuration. The attenuation characteristicof the attenuator 100 shown in FIG. 1 can be obtained by analyzing afrequency characteristic of S12 or S21 that is a diagonal element of ascattering matrix (S matrix). The values of these S12 and S21 changedepending on the parasitic capacitance generated in the circuit and thefrequency. As a result, the attenuation characteristic of the attenuatorobtained by the S12 or the S21 also changes depending on the frequencyof the input signal.

Second Example

FIG. 4 shows an attenuator 400 that employs a variable capacitanceelement 408 as the capacitance element of the attenuator 100 accordingto FIG. 1. Other circuit elements exclusive of the capacitance element408 in the attenuator 400 have the same configuration as that of theattenuator 100.

FIG. 5 shows a change in the attenuation characteristic of theattenuator 400 according to FIG. 4 constituted of MOS transistors 405and 406, a MOS transistor 407, and the variable capacitance element 408,the change appearing when gate widths of the MOS transistors 405 and 406are respectively set to 13.5 [μm], a gate width of the MOS transistor407 is set to 18.2 [μm], and a capacitance value of the variablecapacitance element 408 is varied among 0[fF], 20 [fF], 50[fF], and100[fF]. As apparent from FIG. 5, the capacitance value of the variablecapacitance element 408 is varied so that the attenuation characteristicof the attenuator 400 can be adjusted. Referring to FIG. 5, when thevalue of the variable capacitance element 408 is set to 20[fF], a flatattenuation characteristic can be achieved within the range of up to 5GHz of the frequency of the input signal.

Third Example

FIG. 6 shows an attenuator 600 having a configuration in which twocapacitance elements are further added to the attenuator 400 shown inFIG. 4. More specifically, the attenuator 600 includes MOS transistors605, 606, and 607 and a variable capacitance element 608 that are thesame circuit elements as those in the attenuator 400 of FIG. 4, andfurther includes new capacitance elements 609 and 610. The attenuator600 includes the capacitance elements 609 and 610 in addition to thecapacitance element 608. Thereby, an influence caused when thecapacitance values of these capacitance elements vary at the time ofmanufacturing the attenuator 600 can be reduced, the influence beingapplied to the attenuation characteristic. As described above, theattenuation characteristic of the attenuator can be obtained byanalyzing the frequency characteristic of S12 and S21 that are diagonalelements of the scattering matrix. One of the parameters that exert alarge influence on the values of these S12 and S21 is the capacitancevalue of the capacitance element. Thus, the attenuator 600 is providedwith multiple capacitance elements in the circuit so that a changecaused by variation in the capacitance elements of S12 or S21 that arethe diagonal elements of the scattering matrix can be reduced. S12 andS21 are fractional parameters. Thus, by providing the multiplecapacitance elements in the circuit, terms that change depending on thecapacitance value are included in both a denominator and a numerator ofS12 or S21. Accordingly, even when the capacitance elements 608, 609,and 610 vary at the time of manufacturing the respective capacitancevalues, the change in S12 or S21 is canceled out by a change in thedenominator and a change in the numerator. As a consequence, theattenuation characteristic that can be obtained from S12 or S21 does notlargely change by the variation of the capacitance values of thecapacitance elements 608, 609, and 610 at the time of manufacturing.

Fourth Example

FIG. 7 shows an attenuator 700 in which a parasitic capacitor 711 isgenerated between a gate interconnection and a ground in the attenuator600 shown in FIG. 6. In the attenuator, a parasitic capacitance isactually generated in the gate interconnection and the ground. Circuitelements exclusive of the parasitic capacitor 711 in the attenuator 700are the same as those in the attenuator 600 shown in FIG. 6. Theparasitic capacitor 711 exerts an influence on the attenuationcharacteristic of the attenuator 700. First, a part of an input signalreceived at a terminal 701 flows to a gate interconnection of a MOStransistor 705 via a parasitic capacitance generated between a gateelectrode and a source electrode of the MOS transistor 705 (see, FIG.3). Then, a leakage current that flows to the gate interconnection ofthe MOS transistor 705 flows toward the ground via the parasiticcapacitor 711. Since the shunt current component of the input signalreceived at the terminal 701 increases, the amount of attenuation of thegain of the input signal possibly increases when the frequency of theinput signal is high. However, a change in the attenuationcharacteristic caused by the parasitic capacitance should be avoided asmuch as possible. Accordingly, as a fourth embodiment, FIG. 8 shows anattenuator capable of reducing the leakage current component caused viathe gate interconnection of the MOS transistor. In an attenuator 800shown in FIG. 8, resistances 811, 812, and 813 are respectivelyconnected to gate electrodes of MOS transistors 805, 806, and 807 thatconstitute the attenuator 800. The resistance value of each of theresistances 811, 812, and 813 is 1 [kΩ], for example. The resistance isconnected to each of the gate electrodes of the MOS transistors 805,806, and 807 so that the input signal received at a terminal 801 can beprevented from being leaked via each of the gate interconnections of theMOS transistors 805, 806, and 807. Note that, the attenuator 800 shownin FIG. 8 includes capacitance elements 809 and 810. These capacitanceelements 809 and 810 exhibit the same effect as that of the capacitanceelements 609 and 610 included in the attenuator 600 according to FIG. 6,and are not essential components for preventing the input signalreceived at the terminal 801 from being leaked via each of the gateinterconnections of the MOS transistors 805, 806, and 807. In otherwords, the configuration without including capacitance elements can alsobe employed, as shown in FIG. 12.

Fifth Example

FIG. 9 shows an example that realizes the function of a variablecapacitance element 808 in the attenuator 800 according to FIG. 8 byreplacing it with the MOS transistor. An attenuator 900 shown in FIG. 9includes MOS transistors 907, 910, and 913, and capacitance elements908, 911 and, 914 that are connected in series to these MOS transistors907, 910, and 913, respectively. The voltage to be applied to each ofthe gate electrodes of the MOS transistors 907, 910, and 913 iscontrolled to thereby change the number of the MOS transistors in whichthe source and the drain are conducted with each other, thus realizingthe variable capacitance element. The large number of the MOStransistors among the MOS transistors 907, 910, and 913 in which thesource and the drain are conducted with each other, makes the inputsignal component to be shunted large. This means that the capacitancevalue of the variable capacitance element 808 in FIG. 8 increasesequivalently. On the contrary, the small number of the MOS transistorsamong the MOS transistors 907, 910, and 913 in which the source and thedrain are conducted with each other, makes the input signal component tobe shunted small. This means that the capacitance value of the variablecapacitance element 808 in FIG. 8 decreases equivalently. Note that, inFIG. 9, the three MOS transistors 907, 910, and 913 contribute torealize the equivalent variable capacitance element, and the threecapacitance elements 908, 911 and, 914 contribute to realize theequivalent variable capacitance element. However, the number of the MOStransistors and that of the capacitance elements are not limited tothree. For example, a circuit designer can select the number of the MOStransistors and that of the capacitance elements appropriately dependingon the range of the capacitance value to be changed. Further,capacitance elements 916 and 917 exhibit the same effect as that of thecapacitance elements 609 and 610 included in the attenuator 600according to FIG. 6, and are not essential components to realize theequivalent variable capacitance. Furthermore, resistances 918 and 919exhibit the same effect as that of the resistances 811 to 813 includedin the attenuator 800 according to FIG. 8, and are not essentialcomponents to realize the equivalent variable capacitance.

Sixth Example

It has been described that the attenuation characteristic of theattenuator is adjusted by the variable capacitance element.Additionally, there is an alternative method for adjusting theattenuation characteristic of the attenuator. In this method, the gatevoltage of the MOS transistor is adjusted so that the ON-resistancevalue of the MOS transistor can be adjusted, thereby adjusting theattenuation characteristic of the attenuator. FIG. 10 shows how theattenuation characteristic of the T-type attenuator changes when thegate voltage of a MOS transistor that is connected in shunt in the MOStransistor that constitutes a T-type attenuator is changed. As shown inFIG. 10, the attenuation characteristic of the attenuator can also beadjusted by adjusting the ON-resistance of the MOS transistor.

Although it has been described by using the MOS transistor as thecircuit element that constitutes an attenuator in the present embodimentdescribed above, the attenuator can be formed of a circuit element otherthan the MOS transistor. Thus, the circuit element that constitutes theattenuator should not be limited to the MOS transistor.

It is apparent that the present invention is not limited to the aboveembodiments, but may be modified and changed without departing from thescope and spirit of the invention.

1. An attenuator comprising: a T-type two terminal pair networkincluding first and second terminals, first, second and third circuits,wherein said first terminal receives an input signal to be attenuated,wherein said first circuit is connected between said first and secondterminals, wherein said second circuit is connected between said firstcircuit and said second terminal and is connected to said first circuitvia a node, wherein said third circuit is connected to said node; acapacitor connected to said node, wherein said capacitance value of saidcapacitor is variable; and wherein said first circuit includes a fieldeffect transistor having a gate terminal connected to a resistor.
 2. Theattenuator according to claim 1, wherein said capacitor includes a firstfield effect transistor connected to said node and a first capacitorconnected to said field effect transistor, and further includes a secondfield effect transistor connected to said node and a second capacitorconnected to said second field effect transistor.
 3. The attenuatoraccording to claim 1, further comprising another capacitor connectedbetween said first terminal and said first circuit.